This invention describes a water jet propulsion unit but more particularly a fuel efficient unit which may be used in what are commonly called outboard or stem-drive configurations in motor boats. The device utilises a pair of counter-rotating impellers and seeks to minimise internal frictional losses and the amount of kinetic energy imparted to the jet stream. It may also be described as an axial flow unit which achieves high efficiency gains by acting as a low pressure high mass device rather than a pressure inducing device.
The invention in general terms complies with the design and theoretical requirements described in our U.S. Pat. No. 5,634,831 but the constraints imposed by the desirability of a compact and lightweight outboard or stern-drive configuration introduces a set of parameters which do not apply to our previous designs. The device to be described is thus a low pressure mass transfer device according to our previous invention but with a drive transmission carrier and spring loaded pressure control device placed inside the flow vortex created downstream of the two counter-rotating impellers. In our previous designs the control devices are mounted to the perimeter walls of the nozzle sections. These control devices overcome the difficulties associated with the priming and pressure regulation of the low pressure high mass device so described.
The constraints previously mentioned relate particularly to the requirement to have a right angle drive whereby it is necessary to place a driving shaft across the flow path of the water passing through the inside of the housing of the propulsion unit. Where this shaft passes through the flow path it is critical that it presents a hydrodynamic profile to the impinging flow. The shaft is thus provided with an enclosing structure (vane) of the correct hydrodynamic shape. The section of the driving shaft which passes across the flow path must be of a minimal diameter in order to keep the support vane as narrow as possible and yet have sufficient strength to enable it to reliably accept the load from the driving engine. In this case the enclosing structure or vane is formed into the drive section and is also used to support the drive transmission carrier.
Because of the helical nature of the flow issuing from the upstream impeller it becomes critical, in order to maintain the efficiency of the unit, to also place support structures for the transmission drive carrier in the correct position. ie: the vanes must preferably be placed in an area of axial or linear flow. The reason for this is that any axially aligned structure placed in a helically moving flow is acted upon by the whirl component of that flow. This results in a substantial increase in the level of turbulence generated inside the unit and subsequent interference with the helically moving flow impinging on the downstream impeller. If axial flow is to be achieved at the nozzle outlet any disturbance of the flow on the downstream impeller must be avoided in order for the counter-rotation cancellation effect to remain fully functional. These support structures are also required to have the correct hydrodynamic shape in order to maintain the efficiency of the device. Strict attention to potential frictional losses is particularly important in a low pressure high mass device because through-pump flow velocities are significantly higher than in conventional high pressure statored designs absorbing the same horsepower.
Further, the efficiency of the unit is effected by the relative speed of the two impellers and so for the present invention the relative tip speed of the impellers must be kept in the range of about 0 to 65 meters per second. Impeller tip speed, gearing and engine speed must all be calibrated according to this basic parameter.
In achieving a light weight design which allows ease of disassembly for maintenance purposes the positioning of the impellers in relation to the other mechanical components is also important. The present invention allows this to be achieved without disassembly of the drive components, an important cost saving feature when propulsion units of this type are in routine commercial use. The primary reason for this being the need to maintain impeller clearances inside the pump housing within acceptable tolerances.
In order for the driving components, such as the gears and bearings, to be protected from dirt and other contaminants found in the marine environment, they must be enclosed in a lubricant filled water-tight casing.
A further important constraint is the limited space afforded for the positioning of bevel gears inside the drive transmission carrier whilst also maintaining an axial flow configuration. The specific limitation applies to the size of and therefore strength of the gears necessary for them to accept the input power required. In the case of the stern-drive configuration we largely overcome this by providing for a geared speed reduction in a right angle drive external to the drive transmission carrier as described in the drawings. A one to one ratio in the bevel gears in the drive transmission carrier itself thus makes it possible to maximise gear sizes and thus power input. Where size limitations in a very small construction adversely effects the overall design criteria, the pump casing walls may be diverged or coned slightly to provide sufficient space for the transmission carrier and its components. The shape of the impellers must be altered accordingly with due attention to their relative speed and performance. In this case the upstream impeller is slightly conical in shape, at its periphery, with the downstream impeller also being conical but with approximately one third of the trailing edge of its blades curving smoothly back to parallel with the axis of the unit and the pump casing walls.
Examples of prior art constructions may be seen by reference to the following; U.S. Pat. Nos. 3,082,732; 4,538996; and 4,872,858. In all of these devices the impeller is fixed to the vertically arranged shaft of a conventional outboard layout. Water is drawn through the intake and centrifugaly driven around a horizontally arranged bowl shaped pump housing thence through a nozzle. The numerous directional changes and high pressure design limitations mean these devices allow high fuel consumption and are generally less efficient than both conventional inboard axial flow designs and propellered outboards. A further prior art construction being New Zealand 148,402 shows a pair of axial impellers with stators in the nozzle section as described in New Zealand Patent Specification 123,228. In DE 39 42 672 Al are described several devices containing counter-rotating impellers two of which may be used in outboard configuration where the input shaft passes through the flow inside the pump casing. The propulsion units described are of mixed flow design which the specification states is necessary because the increased diameter of a mixed flow impeller design over that of an axial design enables a drive transmission to be placed within two adjacent impeller hubs.
A number of fundamental design flaws mean that these devices can never be efficient. The reasons for this are as follows;
(i) None can function as a low pressure high mass device as described in our U.S. Pat. No. 5,634,831.
(ii) In the outboard designs described, the insertion of a drive shaft across the flow path, between the two impellers, in a helically moving flow causes unacceptable losses.
(iii) The shape of the downstream impeller in all of the designs conforms to the shape of the nozzle portion of the pump housing , being tapered or frusto conical in shape, where straightening vanes would normally be placed in a conventional design. This part of the design is seriously flawed in that as water passes into the reduced nozzle area it wants to accelerate. However because the impeller blades (which are in the nozzle) are progressively reducing in diameter, along the impeller hub, this means that the circumferential velocity of the impeller tips is also reducing. As a result the amount of kinetic energy imparted to the flow by the blades is also decreasing towards the downstream end of the impeller. The consequence of this is that the impeller blades try to slow the water down while the water itself wants to accelerate. This causes further losses in the overall efficiency of these devices.
(iv) The exposure of the transmission driving components to the harsh conditions found in the marine environment makes this design unsuited for commercial use.
(v) The insertion of a drive shaft between the impellers means that the increased separation distance between the impellers increases in-pump frictional losses due to the fact that the flow between the impellers is helical in motion. Ideally the impellers should be in closest proximity to each other in order to reduce the losses resulting from the water helically spinning inside the pump casing.
(vi) Between impeller insertion of the input shaft increases the complexity of the design in respect of maintenance.
The typical outboard-jet powered motor boat has a high fuel consumption. The primary reasons for this are that outboard jets currently in use do not conform well to propulsion theory in respect of kinetic energy losses to the jet stream and they have high internal frictional losses which arise from poor attention to the design and placement of internal pump structures.